Graciela I. Vujovich

Age Constraints on the Tectonic Evolution and Provenance of the Pie de Palo Complex, Cuyania Composite Terrane, and the Famatinian Orogeny in the Sierra de Pie de Palo, San Juan, Argentina

Abstract New U-Pb age determinations confirm earlier interpretations that the strongly deformed and metamorphosed mafic and intermediate igneous rocks of the Pie de Palo Complex represent a Mesoproterozoic fragment of suprasubduction zone oceanic crust. A gabbroic pegmatite, interpreted to have formed during arc rifting or subsequent back-arc spreading, yielded a U-Pb age of 1204 +5.3/–4.7 Ma. Highly tectonized ultramafic-mafic cumulates, occurring at the structural base of the Pie de Palo Complex and previously interpreted to represent remnants of a primitive arc phase, prior to rifting and back-arc spreading, could not be dated, but should be older than 1204 Ma if these inferences are correct. Tabular, sill-like bodies of leucogabbro/diorite and calc-alkaline tonalite/granodiorite sills yielded ages of 1174±43 and 1169 +8/–7 Ma respectively. They may represent a younger, more evolved arc phase established after arc rifting or a younger, tectonically unrelated Mesoproterozoic arc. SHRIMP-analysis of metamorphic zircon rims with low Th/U ratios in VVL 110 gave a 206Pb/238U age of 455±10 Ma, similar to lower intercept dates determined by discordia lines. Combined, these data indicate that the bulk of the amphibolite facies metamorphism present in the Pie de Palo Complex was generated during the Famatinian Orogeny. Analysis of six single detrital zircon grains in a metasedimentary, quartzofeldspathic garnet-mica schist, tectonically interleaved with the igneous rocks of the Pie de Palo Complex, and tentatively correlated with the Difunta Correa metasedimentary sequence of other workers, yielded three age populations: 1150–1160 Ma; 1050–1080 Ma and 665 Ma, indicating that these sedimentary rocks were deposited during the late Neoproterozoic or Early Paleozoic. In addition, they confirm structural evidence that intercalation of rocks of the Pie de Palo Complex with isolated slivers of these sedimentary rocks is due to tectonic imbrications. These ages are also consistent with a Laurentian provenance, and earlier interpretations that these rocks once represented a sedimentary cover to the Pie de Palo Complex. The zircon population of 1050–1080 Ma could be derived from Grenville-age felsic plutons identified elsewhere in the Pie de Palo Complex by other workers. However, no evidence has been found in our samples for a Grenville-age orogenic event, invoked previously to explain accretion of the oceanic Pie de Palo Complex to Laurentia prior to the late Neoproterozoic/Early Cambrian rifting and drift of Cuyania.

Proterozoic-early Paleozoic ophiolites of the Andean basement of southern South America

We describe a series of Middle Proterozoic to early Paleozoic ultramafic and mafic rocks in the basement of the Central Andes that have different geochemical attributes and paleogeographic settings. Several ophiolitic belts are identified on the basis of lithologic assemblages and geochemical characteristics, but only the most important belts are described here: the Western and Eastern COrdoba (Early Cambrian), Pie de Palo (Middle Proterozoic), Western Precordillera (Middle Ordovician), and eastern Cordillera Frontal (Proterozoic) belts. Most ophiolites in the Central Andean basement belong to one of two types: the harzburgite type and the lherzolite type. For example, in the metamorphic basement in the Western COrdoba belt of the Eastern Sierras Pampeanas, a harzburgite-type ophiolite is associated with important podiform chromite deposits whereas a lherzolite-type ophiolite of the Eastern COrdoba belt was emplaced in a backarc setting. The most ancient ophiolites at these latitudes are exemplified by the Middle Proterozoic Pie de Palo ophiolite, part of the Cuyania terrane, which is a Laurentian-derived block accreted to the protomargin of Gondwana during early Paleozoic time. The ophiolitic assemblage of the Western Precordillera includes serpentinized peridotites, ultramafic cumulates, layered gabbros, diabases, and basaltic pillow lavas. The ophiolites of the eastern Cordillera Frontal are emplaced within three distinctive belts in Precambrian metamorphic rocks. The dominant assemblage of the eastern Cordillera Frontal includes serpentinized peridotites, gabbros, basaltic dikes, and pillow lavas. As a whole, these ophiolite belts represent a complex system of sutures that record the accretionary history of the basement of the Central Andes in several tectonic episodes.

Time constraints on the Early Palaeozoic docking of the Precordillera, central Argentina

Abstract 40Ar/39Ar incremental-release ages have been determined for muscovite and hornblende concentrates prepared from basement rocks of the Sierra de Pie de Palo, in the western Sierras Pampeanas, east of the Early Palaeozoic Precordillera. The basement is mainly represented by variably metamorphosed units, including ophiolites, orthogneisses and schists. Previous U-Pb, Rb-Sr, K-Ar, and 40Ar/39Ar dating of magmatic and metamorphic zircons from the basement has indicated a Middle Proterozoic age. The 40Ar/39Ar plateau ages constrain a series of ductile deformational events that are correlated with development of structural discontinuities in adjacent foreland basins. The following deformational events are postulated: (1) initiation of collision and drowning of the Precordillera platform against Gondwana (470–460 Ma); (2) flexural extension associated with normal faulting due to tectonic loading of the Gondwana margin; and (3) development of a foreland basin (450–430 Ma). These Mid-Ordovician-Silurian events were related to collision of several exotic Early Palaeozoic terranes along the Gondwana margin. The Punta Negra foreland basin developed further west during Early to Mid-Devonian (410–380 Ma) times, and was linked to the beginning of the docking of the Chilenia terrane along the western margin of the Precordillera.

A Laurentian? Grenville-age oceanic arc/back-arc terrane in the Sierra de Pie de Palo, Western Sierras Pampeanas, Argentina

Abstract Whole-rock trace element, field, and petrographical studies indicate that the protoliths of highly deformed metamorphic rocks of the Pie de Palo Complex, Argentina, formed in an oceanic setting like that of SW Pacific supra-subduction complexes (e.g., Lau Basin). Age constraints suggest that this Mesoproterozoic complex, which includes some of the most primitive Grenville-age magmas known on Earth, formed in < 60 million years. Amphibolites (Las Pirquitas/Quemado region) with N-MORB like light-depleted REE patterns and Hf/Ta/Th relations, associated with Fe-rich schists interpreted as hydrothermally altered lavas and gabbros, formed in a back-arc spreading centre. Other ultramafic/mafic units (Quebrada del Gato, Cerros Valdivia/Barboza) formed as arc cumulates, and (central region) as ridge or arc/back-arc lavas (including shoshonite). Intermediate and silicic schists/gneisses are interpreted as arc region greywackes and silicic lavas, and marbles as reef deposits. Quebrada del Gato gneisses with trace elements indicating high pressure residual mineral assemblages are interpreted as younger subduction-related magmas emplaced in a thickened crust. The Pie de Palo Complex probably formed offshore of eastern Laurentia, and subsequently collided with Laurentia east of the Llano uplift, Texas, in Grenvillian times. This block, along with the Precordillera, was rifted from Laurentia in the latest Precambrian, arriving in Gondwana by Ordovician time.

Multiple migmatite events and cooling from granulite facies metamorphism within the Famatina arc margin of northwest Argentina

The Famatina margin records an orogenic cycle of convergence, metamorphism, magmatism, and extension related to the accretion of the allochthonous Precordillera terrane. New structural, petrologic, and geochronologic data from the Loma de Las Chacras region demonstrate two distinct episodes of lower crustal migmatization. The first event preserves a counterclockwise pressure-temperature path in kyanite-K-feldspar pelitic migmatites that resulted in lower crustal migmatization via muscovite dehydration melting at ∼12 kbar and 868°C at 461 ±1.7 Ma. The shape of the pressure temperature path and timing of metamorphism are similar to those of regional midcrustal granulites and suggest pervasive Ordovician migmatization throughout the Famatina margin. One-dimensional thermal modeling coupled with regional isotopic data suggests Ordovician melts remained at temperatures above their solidus for 20–30 Ma following peak granulite facies metamorphism, throughout a time period marked by regional oblique convergence. The onset of synconvergent extension occurred only after regional migmatites cooled beneath their solidus and was synchronous with the cessation of Precordillera terrane accretion at ∼436 Ma. The second migmatite event was regionally localized and occurred at ∼700°C and 12 kbar between 411 and 407 Ma via vapor saturated melting of muscovite. Migmatization was synchronous with extension, exhumation, and strike-slip deformation that likely resulted from a change in the plate boundary configuration related to the convergence and collision of the Chilenia terrane.

U–Pb detrital zircon ages of Upper Jurassic continental successions: implications for the provenance and absolute age of the Jurassic–Cretaceous boundary in the Neuquén Basin

Abstract New U–Pb detrital zircon ages are presented for the Tordillo Formation. The ages indicate that the most important source region of sediment supply was the Jurassic Andean arc (peaks at c. 144, 153 and 178 Ma), although two secondary sources were defined at c. 218 and 275 Ma. Temporal variation in the provenance indicates that at the beginning of the sedimentation, Carboniferous to Lower Jurassic magmatic rocks and Lower Palaeozoic metamorphic rocks were the most important sources. Towards the top, the data suggest that the Andean arc becomes the main source region. The comparison between provenance patterns of the Tordillo Formation and of the Avilé Member (Agrio Formation) showed some differences. In the former, the arc region played a considerable role as a source region, but this is not identified in the latter. The results permit a statistically robust estimation of the maximum deposition age for the Tordillo Formation at c. 144 Ma. This younger age represents a discrepancy of at least 7 Ma from the absolute age of the Kimmeridgian and Tithonian boundary (from the chronostratigraphic timescale accepted by the International Commission of Stratigraphy, IUGS), and has strong implications for the absolute age of the Jurassic–Cretaceous boundary. Supplementary material: Sample coordinates, values of the sandstone compositional framework and U–Pb (LAM-MC-ICP-MS) age measurements of zircons grains are available at http://www.geolsoc.org.uk/SUP18718

Proterozoic metavolcanics from western Sierras Pampeanas terrane, Argentina

Abstract A series of medium grade metamorphic rocks of the western sector of the Sierras Pampeanas Terrane in central western Argentina are represented by amphibolites, gneisses and schists derived from sedimentary as well as from igneous rocks. The metavolcanics consist of amphibolites, quartz-K-feldspar-muscovite schists, and hornblende-biotite and biotite-epidote-plagioclase schists. Based on petrographic and geochemical data they are interpreted as originating as basaltic tholeiites, rhyolites and mesosilicic volcanics. The distribution and geochemical behavior are similar to present day western Pacific lavas, mainly those developed on island arcs or heavily attenuated continental crust. Based on these characteristics, an accretionary tectonic model involving a series of island-arc collisions is proposed for the Proterozoic. The complex Proterozoic tectonic history of the western Sierras Pampeanas has been partially obliterated by the emplacement of the Early Paleozoic magmatic arc rocks.

Crustal shortening, exhumation, and strain localization in a collisional orogen: The Bajo Pequeño Shear Zone, Sierra de Pie de Palo, Argentina

The Bajo Pequeno Shear Zone (BPSZ) is a lower-crustal shear zone that records shortening and exhumation associated with the establishment of a new plate boundary, and its placement in a regional structural context suggests that local- to regional-scale strain localization occurred with progressive deformation. A kilometer-scale field and analytical cross section through the ~80 m thick BPSZ and its adjacent rocks indicates an early Devonian (405–400 Ma) phase of deformation on the western margin of Gondwanan continental crust. The earliest stages of the BPSZ, recorded by metamorphic and microstructural data, involved thrusting of a hotter orthogneiss over a relatively cool pelitic unit, which resulted in footwall garnet growth and reset footwall white mica 40Ar/39Ar ages in proximity to the shear zone. Later stages of BPSZ activity, as recorded by additional microstructures and quartz c-axis opening angles, were characterized by strain localization to the center of the shear zone coincident with cooling and exhumation. These and other data suggest that significant regional tectonism persisted in the Famatinian orogenic system for 60–70 million years after one microplate collision (the Precordillera) but ceased 5–10 million years prior to another (Chilenia). A survey of other synchronous structures shows that strain was accommodated on progressively narrower structures with time, indicating a regional pattern of strain localization and broad thermal relaxation as the Precordillera collision evolved.

Reconstrucción estructural de una sección de la Precordillera Occidental Argentina (31º08’ls-69º24’lo) a partir de datos aeromagnéticos

The magnetic anomaly profile performed along the NE-SW (31o08’LS - 69o24’LO) structural section at the western edge of the Argentine Precordillera, is presented. By the application of ratios between bands, Landsat 7 ETM satellite images were processed as well as aeromagnetic data by filtering methods (reduction to the pole and calculation of the residual magnetic anomaly) and emphasized magnetic anomaly (analytical signal). Two lithofacies within the Don Polo Formation (lithofacies I at the base and II at the top of the sequence) were recognised. The main structure of Villa Corral mountain range is defined by a synclinal fold with a dipping axis to the SE and similar smaller-scale folds associated, which altogether are fragmented by two landslide systems: the Pre- Andean one, oriented NW-SE inclined fault planes to the NE and top-to-the SW motion; the other, Andean, with N-S direction and inclined fault plane to the W with top-to-the E motion. The analytical signal profile shows the importance of magnetic contrasts produced by major landslides that overlap clastic metasedimentary rocks of different stratigraphic levels intruded by seams of igneous acidic rock layers. Thus, magnetic methods allow adjusting the distribution of underground units according to the relative displacement of fault blocks.